External Rewriting for Skeptical Proof Assistants

This paper describes a fibrational categorical semantics for the modal necessity-only fragment of constructive modal type theory, both with and without dependent types. Constructive type theory does not usually discuss logical modalities, and modalities tend to be mostly studied within classical logic, not within type theory. But modalities should be very useful in type theory, as they are very useful in modelling theoretical computing systems. Providing constructive versions of modal logics and their associated Curry-Howard modal type theories is also a very productive program, e.g. helpful when dealing with computational effects, staged computation, and functional reactive types, for example. There seems to be renewed interest in the notion of constructive modal type theory (and in notions of linear type theory), in part because of the interest in homotopy type theory. The modal type theory presented here uses dependent types, in the style of Ritter's categorical models of the Calculus of Constructions. To build up to these, we first discuss the kinds of constructive modal type theory in the literature. Then we provide a non-dependent modal type theory, introduced in previous work, that we generalize to dependent types in the following section. Dependent type theories are usually but not always given categorical semantics in terms of fibrations. We provide semantics in terms of fibrations for both the non-dependent and the dependent type systems discussed and prove them sound and complete, thereby providing evidence that the type theory is meaningful. These fibrational models should be also applicable to the homotopy type theory setting.

The project entitled “Formalization of Formal Topology by means of the interactive theorem prover Matita” is an official bilateral project between the Universities of Padova and Bologna, funded by the former, active from March 2008 until August 2010. The project aimed to bring together and exploit the synergic collaboration of two communities of researchers, both centered around constructive type theory: on one side the Logic Group at the University of Padova, focused on developing formal, pointfree topology within a constructive and predicative framework; on the other side, the Helm group at the University of Bologna, developing the Matita Interactive Theorem Prover [2], a young proof assistant based on the Calculus of Inductive Constructions as its logical foundation.

The authors present a proof development system called PowerEpsilon, based on a constructive type theory which can be used as a formal program development system for deriving a program from a specification. PowerEpsilon is a polymorphic language based on Martin-Lof's type theory and the calculus of constructions. In PowerEpsilon, the concept of limit of type universe hierarchies and a scheme for inductive define types are introduced. The system can be used as both a programming language with a very rich set of data structures and a metalanguage for formalizing constructive mathematics. The system has been implemented. A programming exercise is given to show how the system works. >

Quotient topologies in constructive set theory and type theory

Proof assistants based on dependent type theory are closely related to functional programming languages, and so it is tempting to use them to prove the correctness of functional programs. In this paper, we show how Agda, such a proof assistant, can be used to prove theorems about Haskell programs. Haskell programs are translated into an Agda model of their semantics, by translating via GHC's Core language into a monadic form specially adapted to represent Haskell's polymorphism in Agda's predicative type system. The translation can support reasoning about either total values only, or total and partial values, by instantiating the monad appropriately. We claim that, although these Agda models are generated by a relatively complex translation process, proofs about them are simple and natural, and we offer a number of examples to support this claim.

QPC 2: A constructive calculus with parameterized specifications

We report our first experience in enhancing automated grading in an undergraduate programming course using formal verification. In our experiment, we deploy a program verifier to check the equivalence between student submissions and reference solutions, alongside the existing, testing-based grading infrastructure. We find that we can use the conservative nature of program equivalence checking to our advantage. We were able to use such equivalence to differentiate student submissions according to their high-level program structure, in particular their recursion pattern, even when their input-output behaviour is identical. Consequently, we achieve (1) higher confidence in correctness of idiomatic solutions but also (2) more thorough assessment of solution landscape that reveals solutions beyond those envisioned by instructors.

The open calculus of constructions integrates key features of Martin-Löf's type theory, the calculus of constructions, membership equational logic, and rewriting logic into a single uniform language. The two key ingredients are dependent function types and conditional rewriting modulo equational theories. We explore the open calculus of constructions as a uniform framework for programming, specification and interactive verification in an equational higher-order style. By having equational logic and rewriting logic as executable sublogics we preserve the advantages of a first-order semantic and logical framework and we provide a foundation for a broad spectrum of applications ranging from what could be called executable mathematics, involving symbolic computations and logical proofs, to software and system engineering applications, involving symbolic execution and analysis of nondeterministic and concurrent systems.

The calculus of constructions as a framework for proof search with set variable instantiation

In this article we continue the work started in [Ernesto Copello, Álvaro Tasistro, Nora Szasz, Ana Bove, and Maribel Fernández. Alpha-structural induction and recursion for the λ-calculus in constructive type theory. Electronic Notes in Theoretical Computer Science, 323:109–124, 2016], deriving in Constructive Type Theory new induction principles for the λ-calculus, using (the historical) first order syntax with only one sort of names for both bound and free variables, and with α-conversion based upon name swapping. The principles provide a flexible framework for mimicking pen-and-paper proofs within the rigorous formal setting of a proof assistant. We here report on one successful application, namely a complete proof of the Church-Rosser Theorem. The whole development has been machine-checked using the system Agda [Ulf Norell. Towards a Practical Programming Language Based on Dependent Type Theory. PhD thesis, Department of Computer Science and Engineering, Chalmers University of Technology, September 2007].

The aim of this book is to show how constructive type theory, in the form developed by Per Martin-Löf, helps us to understand the structure of natural language. The study of similar questions within simple (=classical) type theory was inaugurated by Frege and given a standard of technical detail by Montague. The point of shifting to constructive type theory is not in the constructivist criticism against classical reasoning, but in the positive observation that constructive type theory has new things to say. Some of the problems to which grammars based on classical type theory have not found satisfactory solutions will be shown to require precisely these new things. These problems appear in a simple form in what is known, since Geach, as donkey sentences. But they are repeated in other guises, for example, in the functioning of tenses, and in a larger scale, such as in the progressive structure of texts, and in the significance of context in communication.

Martin-Lof’s Constructive Type Theory (CTT) is a formal language developed in order to reason constructively about mathematics. It is thus a formal language conceived primarily as a tool to reason with rather than a formal language conceived primarily as a mathematical system to reason about. Constructive Type Theory is therefore much closer in spirit to Frege’s ideography and to the language of Russell and Whitehead’s Principia Mathematica than to the majority of logical systems (“logics”) studied by contemporary logicians. Since CTT is designed as a language to reason with, much attention is paid to the explanation of basic concepts. This is perhaps the main reason why the style of presentation of CTT differs somewhat from the style of presentation typically found in, for instance, ordinary logic textbooks. For those new to the system it might be useful to approach an introduction, such as the one given below, more as a language course than as a course in mathematics.

Taking Per Martin-Löf’s constructive type theory as a starting-point a theory of assertion is developed, which is able to account for the epistemic aspects of the speech act of assertion, and in which it is shown that assertion is not a wide genus. From a constructivist point of view, one is entitled to assert, for example, that a proposition A is true, only if one has constructed a proof object a for A in an act of demonstration. One thereby has grounded the assertion by an act of demonstration, and a grounding account of assertion therefore suits constructive type theory. Because the act of demonstration in which such a proof object is constructed results in knowledge that A is true, the constructivist account of assertion has to ward off some of the criticism directed against knowledge accounts of assertion. It is especially the internal relation between a judgement being grounded and its being known that makes it possible to do so. The grounding account of assertion can be considered as a justification account of assertion, but it also differs from justification accounts recently proposed, namely in the treatment of selfless assertions, that is, assertions which are grounded, but are not accompanied by belief.

In a previous work, we proved that almost all of the Calculus of Inductive Constructions (CIC), the basis of the proof assistant Coq, can be seen as a Calculus of Algebraic Constructions (CAC), an extension of the Calculus of Constructions with functions and predicates defined by higher-order rewrite rules. In this paper, we prove that CIC as a whole can be seen as a CAC, and that it can be extended with non-strictly positive types and inductive-recursive types together with non-free constructors and pattern-matching on defined symbols.KeywordsInduction HypothesisType TheoryPredicate SymbolProof AssistantInductive TypeThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Orthogonality is a discipline of programming that in a syntactic manner guarantees determinism of functional specifications. Essentially, orthogonality avoids, on the one side, the inherent ambiguity of non determinism, prohibiting the existence of different rules that specify the same function and that may apply simultaneously (non-ambiguity), and, on the other side, it eliminates the possibility of occurrence of repetitions of variables in the left-hand side of these rules (left linearity). In the theory of term rewriting systems (TRSs) determinism is captured by the well-known property of confluence, that basically states that whenever different computations or simplifications from a term are possible, the computed answers or the obtained reduced terms should coincide. Although the proof is technically elaborated, confluence is well-known to be a consequence of orthogonality. Thus, orthogonality is an important mathematical discipline intrinsic to the specification of recursive functions that is naturally applied in functional programming and specification. Starting from a formalization of the theory of TRSs in the proof assistant PVS, this work describes how confluence of orthogonal TRSs is being formalized in this proof assistant. Substantial progress has been done in this research, obtaining until now complete formalizations for some similar, but restricted properties, such as a complete formalization for the property of confluence of non-ambiguous and (left and right) linear TRSs. 1